kernel - Add frontend cache for cache_findmount()

* When a name lookup crosses a mount point boundary it must call
  cache_findmount() to locate the mount linkage.  This linkage is
  not stored in the vp or ncp because there is a 1:N relationship
  between vp/ncp and possible mounts due to DragonFly's ability
  to do arbitrary nullfs mounts in the topology.

* The mountlist scan requires an exclusive token to deal with ripouts
  during the scan.  This creates a bottleneck when highly parallel
  filesystem operations are being run on the machine and use mount-crossing
  paths or absolute paths.

* The frontend cache is able to use a shared spinlock for the fast path,
  and implements a simple non-chained linear array hashed by pointer
  values.
Merge branch 'master' into gsocdrm_github